1. Technical Field
The present invention relates to a static electricity protection circuit, an electro-optic device including the static electricity protection circuit and an electronic device including the electro-optic device.
2. Related Art
Active driving type liquid crystal devices as electro-optic devices include pixels for modulating light, semiconductor circuits (a scanning line driving circuit, a data line driving circuit and the like) for driving the pixels, and the like. In such a liquid crystal device, sometimes, transistors constituting the pixels and the semiconductor circuits are subjected to a non-recoverable electrostatic damage due to static electricity, and thus, it is important to take measures against static electricity for restricting the influence of static electricity. For example, in JP-A-2006-18165, a liquid crystal device provided with an electrostatic protection circuit (a static electricity protection circuit) has been proposed.
FIG. 16 is a circuit diagram of a static electricity protection circuit described in JP-A-2006-18165. As shown in FIG. 16, a static electricity protection circuit 500 described in JP-A-2006-18165 includes a p-type transistor 504 and an n-type transistor 505. The source and the gate of the p-type transistor 504 are connected to a high electric potential wiring 502 and are supplied with an electric potential VH. The source and the gate of the n-type transistor 505 are connected to a low electric potential wiring 503 and are supplied with an electric potential VL which is lower than the electric potential VH. The drain of the p-type transistor 504 and the drain of the n-type transistor 505 are connected to a signal wiring 501.
When the electric potential of the signal wiring 501 is within a range between the electric potential VL and the electric potential VH, the p-type transistor 504 and the n-type transistor 505 are in off-state, and any electric interference among the signal wiring 501, the high electric potential wiring 502 and the low electric potential wiring 503 does not occur, so that the liquid crystal device operates normally. When the electric potential of the signal wiring 501 deviates from the range between the electric potential VL and the electric potential VH due to static electricity, one of the p-type transistor 504 and the n-type transistor 505 becomes in on-state (in a conducting state). For example, when the electric potential of the signal wiring 501 becomes higher than VH due to static electricity, the p-type transistor 504 becomes in on-state. When the electric potential of the signal wiring 501 becomes lower than VL due to static electricity, the n-type transistor 505 becomes in on-state. In this way, when the electric potential of the signal wiring 501 varies due to static electricity, a path leading from the signal wiring 501 to either the high electric potential wiring 502 or the low electric potential wiring 503 becomes in a conducting state. Further, charges caused on the signal wiring 501 by static electricity are distributed to a path leading to the high electric potential 502 or a path leading to the low electric potential 503, whichever has become in a conducting state, so that an amount of the variation of the electric potential of the signal wiring 501 due to static electricity becomes small. Since an amount of the variation of the electric potential of the signal wiring 501 due to static electricity becomes small, a non-recoverable electrostatic damage (electrostatic destruction) on semiconductor circuits connected to the signal wiring 501 becomes unlikely to occur.
As described above, in the static electricity protection circuit 500 described in JP-A-2006-18165, when positive charges are caused on the signal wiring 501 by static electricity, the p-type transistor 504 becomes in on-state, and the positive charges caused on the signal wiring 501 are distributed (discharged) to a path leading to the high electric potential wiring 502 via the p-type transistor 504, so that an amount of a variation of the electric potential of the signal wiring 501 due to static electricity becomes small. Nevertheless, the mobility of carriers (holes) of the p-type transistor 504 is smaller than the mobility of carriers (electrons) of the n-type transistor 505, and thus, in a case of the p-type transistor 504, an electric current (charges) is (are) more unlikely to flow, as compared with a case of the n-type transistor 505. For this reason, when a large number of positive charges are caused on the signal wiring 501 by static electricity, a sufficient number of the positive charges are not distributed (discharged) to the path leading to the high electric potential wiring 502 via the p-type transistor 504 and an amount of the variation of the electric potential of the signal wiring 501 becomes large, so that a non-recoverable electrostatic damage (electrostatic destruction) on semiconductor circuits, the n-type transistor 505 in a non-conducting state and the like which are connected to the signal wiring 501 is likely to occur.